Covering for photothermal conversion

ABSTRACT

An infrared radiation reflecting substrate, having deposited thereon a first layer formed from a metal or an alloy, and a second layer deposited on the first layer and consisting of a solar radiation absorbing amorphous material, such as amorphous carbon. The substrate can be formed from an infrared radiation reflecting layer deposited on a primary substrate.

BACKGROUND OF THE INVENTION

The present invention relates to a covering for photothermal conversionand more particularly applies to the photothermal conversion of solarenergy.

It is known that so-called "selective" surfaces are being increasinglyused for the photothermal conversion of solar energy, because they makeit possible to significantly improve conversion efficiencies. Thesesurfaces are such that they heat by absorbing incident solar radiationin the same way as a black body, but unlike the latter, they only emitvery little infrared radiation, so that their heat losses are minimized.Among the different known methods for producing them, that which is mostfrequently used at present consists of depositing a thin layer of amaterial absorbing solar radiation on an only slightly emissive layer,i.e. which reflects infrared radiation.

At present there are two main groups of methods used for depositing thethin layer of absorbent materials, namely liquid phase depositionmethods (chemical and electrolytic deposits, deposits by immersion,etc.), and vapor phase deposition methods (vacuum evaporation, cathodicsputtering, etc).

The methods of the second group are more difficult to carry out thanthose of the first group, but nevertheless offer the possibilities ofproducing composite materials, which would be difficult or evenimpossible to obtain by using liquid phase deposition methods.

However, the methods of the first and second groups have one point incommon, namely with said methods, an attempt is generally made toproduce a thin absorbent layer of the "cermet" type, which is a veryfine dispersion of a phase having a metallic nature in a matrix having adielectric nature and in order to increase still further the absorbingproperties of the cermet, its composition is made variable in itsthickness. Thus, attempts are made to obtain a cermet having a metallicnature at the interface with the infrared radiation reflecting layer andwith a dielectric nature at the interface with the ambient medium (air)to obtain a so-called "graded" cermet.

The optical properties of the numerous cermets are now well known andhave been widely publicized in the literature. For example, referencecan be made to cermets obtained by the reactive cathodic sputtering of astainless steel target in a residual atmosphere of argon and acetylene.Cermets of this type are, for example, envisaged in U.S. Pat. No.4,309,261 and in a communication entitled "In line production system forsputter deposition of graded index solar absorbing films", by D. R.McKenzie et al., 8th International Vacuum Congress, Cannes, September1980.

Apart from the fact that reactive cathodic sputtering is much moredifficult to control than non-reactive cathodic sputtering due, interalia, to the pressure gradient of the reactive gas which has to bemaintained in the sputtering chamber, the layers of the type referred toin the aforementioned paragraph and obtained by reactive cathodicsputtering, contain a large proportion of hydrogen, as has been statedin the article entitled "Properties of hydrogenated carbon filmsproduced by reactive magnetron sputtering", by D. R. McKenzie et al.,published in Solar Energy Materials, 6, 1981, pp. 97-106.

The presence of hydrogen in the deposited layer is prejudicial to theproduction of a vacuum transducer for photothermal conversion, becauseit makes the degassing operations which have to be carried out duringits production more difficult. Moreover, the desorption of this hydrogenduring heat treatment seems to be one of the most important reasons forthe deterioration in the optical properties of graded cermets producedfrom stainless steel and carbon.

Coverings for photothermal conversion are also known, which are obtainedby a method consisting of depositing a graphite layer on an infraredradiation reflecting layer,, the latter being itself deposited on asubstrate and which can be of copper, silver, nickel or titanium. Thismethod is described in the article entitled "Effect of substrate ongraphite and other solar selective surfaces", by D. R. McKenzie,published in Applied Optics, vol. 17, no. 12, 1978, pp. 1984 to 1988. Itis much simpler than the reactive cathodic sputtering method and cantherefore be much more easily controlled, because it only involves thesuperimposing of two elementary homogeneous layers. Moreover, theselayers are free from prejudicial foreign atoms, such as hydrogen atoms,due to the procedure used for depositing the graphite layer, namelyvacuum evaporation by an electron gun.

Unfortunately, the graphite layers obtained by this method are notsufficiently absorbent to be suitable for industrial applications. Thus,their solar absorption factor is only 0.70 for graphite deposited oncopper and 0.80 for graphite deposited on titanium.

SUMMARY OF THE INVENTION

The present invention relates to a covering for photothermal conversion,which does not suffer from the disadvantages of the known coveringsdescribed hereinbefore. Thus, it can be produced in a simple manner andhas thermally stable layers, which are free from hydrogen and whosesolar absorption factor can be equal to or higher than 0.90.

More specifically, the present invention relates to a covering forphotothermal conversion, wherein it comprises an infrared radiationreflecting substrate, a first layer deposited on this substrate andconstituted by at least one metallic compound, and a second layerdeposited on the first layer, said second layer being constituted by anamorphous material and being able to absorb solar radiation.

According to another feature of the covering according to the invention,the substrate is formed by an infrared radiation reflecting layer and aprimary substrate, the infrared radiation reflecting layer beingdeposited on the primary substrate.

In other words, the first and second layers can be deposited on a solidinfrared radiation reflecting substrate, or on an infrared radiationreflecting layer, previously deposited on the socalled primarysubstrate.

It is only as a result of the use of a metallic layer, between theinfrared radiation reflecting substrate and the absorbent layer (secondlayer), and the use of such a non-crystalline, amorphous, absorbentlayer (unlike graphite, which has a crystalline structure), that it ispossible to obtain a covering having a high reflection factor, whilststill retaining a very low emission factor in the infrared. The infraredradiation reflecting substrate can, for example, consist of copper,silver or gold.

According to another special feature of the covering according to theinvention, the first layer is produced from at least one of thematerials taken in the group including transition metals and theiralloys. The first layer is, for example, obtained from a stainlesssteel, due to its ease of production and low cost.

According to yet another feature, the thickness of the first layer isapproximately a few dozen nanometers, e.g. approximately 10 to 50 nm.

According to another feature, the second layer is an amorphous carbonlayer.

According to a preferred feature, the thickness of the second layer isequal to or greater than that of the first layer, e.g. has a thicknessof approximately 50 to 150 nm.

According to yet another feature, the first and second layers aredeposited by cathodic sputtering.

Finally, according to yet another feature, when the substrate isconstituted by the infrared radiation reflecting layer on the primarysubstrate, the infrared radiation reflecting layer and the first andsecond layers are deposited by cathodic sputtering.

DESCRIPTION OF THE DRAWING AND PREFERRED EMBODIMENTS

Other features and advantages of the covering for photothermalconversion according to the invention will become more apparent from thefollowing description relative to a non-limitative embodiment and withreference to the attached drawing, which diagrammatically shows part ofa covering according to the invention.

In this embodiment, the covering according to the invention comprises aninfrared radiation reflecting layer 1, deposited on a primary substrate2, so as to form an infrared radiation reflecting substrate 3 and afirst layer 4, together with a second layer 5, successively deposited onthe infrared radiation reflecting layer.

The different layers are successively deposited on the primary substrateby cathodic sputtering in an inert gas, such as argon. Thus, onto theprimary substrate, formed for example from a borosilicate glass(marketed under the trademark PYREX), there is deposited the infraredradiation reflecting layer, which is a thin layer made e.g. from copper,in preference to silver or gold, which are expensive metals. This isfollowed by the successive deposition of a first metallic layer, then asecond amorphous layer, which are very thin and homogeneous and serve toform a very selective covering, combined with the copper layer.

The first layer is obtained by the deposition of a very thin metallicfilm having a thickness between 10 and 50 nm. To achieve this, thematerial used is, for example, stainless steel, due to the fact that itis simple to produce and inexpensive.

The second layer is then obtained by the cathodic sputtering of a carbontarget in a residual argon atmosphere. The second amorphous carbon layerpreferably has a thickness which is at least equal to that of the firstlayer. For example, the thickness of the second layer is between 50 and150 nm.

Apart from a high thermal stability, the covering obtained has veryinteresting optical properties for the photothermal conversion of solarenergy. Thus, it is possible to obtain, by operating in the mannerdescribed hereinbefore, coverings having an absorption factor equal toor higher than 0.90, but whose emission factor remains equal to or below0.05.

As an illustrative and non-limitative example, the followingexperimental conditions can be used for producing with the aid of acathodic sputtering installation for producing deposits on cylindricalsubstrates, whereby said known installation is called a cylindricalmagnetron, a covering according to the invention on a 28 mm diameterPYREX tube used as the primary substrate.

Notations:

(1) Type of cathode

(2) Argon pressure

(3) Cathode current density

(4) Sputtering voltage

(5) Sputtering time

(6) Deposite thickness

    ______________________________________                                        Deposition of the infrared radiation reflecting                               layer:                                                                        (1)   Of copper                                                               (2)   Between 6.10.sup.-3 and 2.10.sup.-2 Torr, optimum 10.sup.-2 Torr        (3)   Between 5 and 8 mA/cm.sup.2 optimum 6.0 mA/cm.sup.2                     (4)   optimum 500 V                                                           (5)   Between 45 and 65 s optimum 55 s                                        (6)   Between 350 and 550 nm optimum 450 nm                                         Deposition of the first layer:                                          (1)   Of stainless steel                                                      (2)   Between 6.10.sup.-3 and 2.10.sup.-2 Torr, optimum 10.sup.-2 Torr        (3)   Between 5 and 20 mA/cm.sup.2 optimum 10 mA/cm.sup.2                     (4)   optimum 580 V                                                           (5)   Between 2.5 and 12 s optimum 5.5 s                                      (6)   Between 10 and 50 nm optimum 22.5 nm                                          Deposition of the second layer; produced from                                 carbon:                                                                 (1)   Of carbon                                                               (2)   Between 6.10.sup.-3 and 2.10.sup.-2 Torr, optimum 10.sup.-2 Torr        (3)   Between 5 and 8 mA/cm.sup.2 optimum 6 mA/cm.sup.2                       (4)   optimum 650 V                                                           (5)   Between 55 and 165 s optimum 90 s                                       (6)   Between 50 and 150 nm optimum 80 nm                                     ______________________________________                                    

Under the aforementioned conditions and using a method which is verysimple for the expert to perform, it is possible to obtain a coveringwith a solar absorption factor of 0.91 and an emission factor of 0.04.

Under the same experimental conditions as hereinbefore, it is possibleto produce the covering by directly placing the stainless steel layerand the amorphous carbon layer on a polished infrared radiationreflecting tube, made e.g. from copper, without using the primary PYREXsubstrate.

Obviously, the invention does not only relate to coverings forphotothermal conversion, deposited on tubular or cylindrical substrates.It also relates to coverings, produced by cathodic sputtering, onsubstrates having different shapes and forms, by using adapted, knownelectrodes. For example, in the case of a planar substrate, the latteris held by a substrate holder and a cathode is arranged in parallel andfacing the substrate.

Finally, the invention is not limited to the coverings obtained bycathodic sputtering. For the deposition of the absorbent layer, e.g. ofamorphous carbon, it is possible to use all known methods, such asevaporation or chemical deposition in the vapor phase in the presence ofa plasma.

What is claimed is:
 1. A covering for photothermal coversion, comprisingan infrared radiation reflecting substrate, a first layer deposited onsaid substrate and consisting of at least one metal or alloy of saidmetal, and a second layer deposited on the first layer, said secondlayer consisting of an amorphous material capable of absorbing solarradiation.
 2. The covering according to claim 1, wherein the substratecomprises an infrared radiation reflecting layer and a primarysubstrate, the infrared radiation reflecting layer being deposited onthe primary substrate.
 3. The covering according to claim 1, wherein theinfrared radiation reflecting layer is a metal selected from the groupconsisting of copper, silver and gold.
 4. The covering according toclaim 1, wherein the first layer is made of at least one of thematerials selected from the group consisting of transition metals andtheir alloys.
 5. The covering according to claim 4, wherein the firstlayer is made of stainless steel.
 6. The covering according to claim 1,wherein the thickness of the first layer is approximately 10 to 50 nm.7. The covering according to claim 1, wherein the second layer is anamorphous carbon layer.
 8. The covering according to claim 1, whereinthe thickness of the second layer is equal to or greater than that ofthe first layer.
 9. The covering according to claim 8, wherein thethickness of the second layer is approximately 50 to 150 nm.
 10. Thecovering according to claim 1, wherein the first and second layers aredeposited by cathodic sputtering.
 11. The covering according to claim 2,wherein the infrared radiation reflecting layer and the first and secondlayers are deposited by cathodic sputtering.